Cytochrome bc1 reductase is an integral membrane protein complex, over 240 kDa in weight and composed of 11 different subunits. Biologically, this complex is quite important since it is a key link in the mitochondrial respiratory cycle, coupling the electron transfer from ubiquinol to cytochrome c with the formation of the proton gradient across the inner mitochondrial membrane. The complexity as well as the rather large size of the protein complex requires the use of synchrotron sources for all data collection and screening studies. We are requesting beam time on beamline 1-5 for screening of heavy atom derivatives. This beamline is perfectly suited for this purpose with its capacity to go to long camera distances of more than 500 mm. It also has a tunable wavelength source so that we can maximize the anomalous signal for the different heavy atom derivatives.